Granular metallic iron

ABSTRACT

Metallic iron nuggets made by reducing-melt of a material containing a carbonaceous reductant and a metal-oxide-containing material, the metallic iron nuggets comprising at least 94% by mass, hereinafter denoted as “%”, of Fe and 1.0 to 4.5% of C, and having a diameter of 1 to 30 mm are disclosed.

TECHNICAL FIELD

The present invention relates to metallic iron nuggets made byreducing-melt of a material containing iron oxide, such as iron ore, anda carbonaceous reductant, such as coke, the metallic iron nuggets havinga high Fe purity, specified C, S, Si, and Mn contents, and a specifieddiameter.

BACKGROUND ART

A direct iron-making process for making reduced iron by direct reductionof an iron oxide source such as iron ore using a carbonaceous substanceor a reducing gas has long been known. Extensive research has beenconducted as to the specifics of the reducing process and continuousreduction equipment.

For example, Japanese Unexamined Patent Application Publication No.11-337264 discloses a rotary hearth that allows efficient continuousproduction of reduced iron, in which, during reduction by heating ofgreen pellets prepared by solidifying a mixture of an iron oxide sourcesuch as steelmaking dust or fine ore and a carbonaceous substance usinga binder, explosions which occur when undried green pellets are rapidlyheated are prevented due to installation of a preheating zone.

In the technology, including the above-described technology, for makingmetallic iron by heating and reducing compacts containing an iron oxidesource and a reductant, a considerable amount of a slag componentbecomes mixed in the resulting metallic iron due to the use of the ironore or the like. In particular, in a method for making sponge metalliciron, the Fe purity is drastically low because the separation of theslag component that became mixed in the metallic iron is difficult.Thus, a preliminary treatment for removing this considerable amount ofslag component is required when these materials are used as an ironsource. Moreover, nearly all of the metallic iron obtained by a knowndirect iron-making process is sponge-shaped, and thus the handlingthereof as an iron source is difficult since such metallic iron isfragile. In order to actually use such metallic iron as a material formaking iron, steel, or alloy steel, a process such as a secondaryprocess to make briquettes therefrom is required, and the expenses foradditional equipment therefor are considerable.

Japanese Unexamined Patent Application Publication No. 9-256017discloses a method for making metallic iron nuggets having a highmetallization ratio, the method including heating and reducing compactscontaining iron oxide and a carbonaceous reductant until a metallic ironsheath is formed and substantially no iron oxide is present in the innerportion while forming nuggets of the produced slag in the inner portion,continuing heating so as to allow the slag inside to flow outside of themetallic iron sheath so as to separate the slag, and further performingheating so as to melt the metallic iron sheath.

In the known processes, including these conventional techniques, formaking metallic iron nuggets, no technology capable of efficientlymaking metallic iron having a diameter within a predetermined rangewhile fully considering the quality and handling convenience ofmaterials for making iron, steel, or iron alloy has been established. Asfor the purity of the metallic iron nuggets, although high-puritymetallic iron nuggets with a low contaminant content are naturallypreferred, no specific idea for specifying the optimum carbon content inthe metallic iron nuggets used as the material for iron making andsteelmaking has been formulated. Moreover, no specific manufacturingtechnology for controlling the carbon content within a predeterminedrange has been established.

Furthermore, when metallic iron is made by reducing iron oxide such asore, coke or a coal powder is generally used as the reductant. However,these reductants normally have a high sulfur (S) content. Since thereductant becomes mixed in the metallic iron produced, the resultingmetallic iron nuggets normally have a high S content. Accordingly, themetallic iron nuggets must be subjected to desulfurization before theyare actually used as the material for making iron or steel. This is alsoone of the main reasons for the degradation in quality of the metalliciron nuggets.

Accordingly, in order to make metallic iron nuggets of high value by areducing-melt process, it is not sufficient to merely hope to increasethe purity. A technology that can reliably make metallic iron, in whichthe contaminant content, such as a sulfur content, is specified and thesize of which is optimized in view of production possibility andhandling quality, the technology also being capable of satisfying thedemands in the market such as a greater flexibility in the choice ofmaterial for making iron, steel, or various alloy steels, and reductionof the cost required for making iron or steel using, for example, anelectric furnace, is required to be established.

The present invention is developed based on the above-describedbackground. An object of the present invention is to provide metalliciron nuggets of stable quality that have an optimum size in view of theoverall production possibility and handling quality as an iron source,and in which the contaminant content of the metallic iron nuggets, suchas carbon and sulfur contents, is specified. The metallic iron nuggetsof the present invention can thus satisfy the demands in the market suchas a greater flexibility in the choice of material for making metalliciron and a reduction of the cost required for making iron or steelusing, for example, an electric furnace.

DISCLOSURE OF INVENTION

Metallic iron nuggets of the present invention that overcome theabove-described problems are metallic iron nuggets having an Fe contentof 94% (percent by mass, contents of components are in terms of percentby mass) or more, a C content of 1.0 to 4.5%, a S content of 0.20% orless, and a diameter of 1 to 30 mm, the metallic iron nuggets being madeby reducing-melt of a material containing a carbonaceous reductant andan iron-oxide-containing material.

The metallic iron nuggets of the present invention need not bespherical. Granular substances having an elliptical shape, an ovalshape, and slightly deformed shapes thereof are also included in themetallic iron nuggets of the present invention. The diameter of thenuggets ranging from 1 to 30 mm is determined by dividing the total ofthe lengths of the major axis and the minor axis and the maximum andminimum thicknesses of a nugget by 4.

Preferably, the metallic iron nuggets further include 0.02 to 0.50% ofSi and less than 0.3% of Mn.

The metallic iron nuggets are prepared by heating the material so as toreact a metal oxide contained in the material with the carbonaceousreductant and a reducing gas produced by such a reaction and to reducethe metal oxide in the solid state, and further heating the resultingreduced iron in a reducing atmosphere so as to carburize and melt theresulting reduced iron and allow the reduced iron to cohere whileexcluding any by-product slag. During this process, a CaO source isadded to the material to adjust the basicity of the slag components inthe material, i.e., CaO/SiO₂, within the range of 0.6 to 1.8. In thismanner, sulfur contained in the material can be efficiently captured bythe slag produced during reducing-melt, and metallic iron nuggets havinga S content of 0.08% or less can be obtained.

The amount of the carbonaceous reductant is adjusted so that theremaining carbon content during the step of reducing-melt of thematerial is in the range of 1.5 to 5.0% when the metallization ratio ofthe metallic iron nuggets after the solid reduction is 100%. In thismanner, the resulting carbon content can be controlled within theabove-described range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory schematic view showing an example ofreducing-melt equipment for making metallic iron nuggets of the presentinvention.

FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1.

FIG. 3 is an explanatory cross-sectional view in which FIG. 1 isdeveloped in the longitudinal direction.

FIG. 4 is a graph showing the transitions of the atmosphere temperature,the temperature of material compacts, the reduction ratio, and theamount of CO and CO₂ gasses throughout a solid-reduction period and amelting period when a two-stage heating process is employed in thepresent invention.

FIG. 5 is a graph showing the transitions of the residual Fe content andthe metallization ratio of the metal oxide in the material compactsthroughout the solid-reduction period and the melting period.

FIG. 6 is a graph showing the relationship between the residual carboncontent in the reduced iron when the metallization ratio is 100% and theresidual carbon content of the end product metallic iron nuggets.

FIG. 7 is a graph showing the relationship between the metallizationratio and the reducing degree.

FIG. 8 is a graph showing a change in the reducing degree of anatmosphere gas and in the temperature of the interior of the materialcompacts when a coal powder is used as an atmosphere adjustor and whenthe coal powder is not used as an atmosphere adjustor.

FIG. 9 is a photograph showing the state of metallic iron and slagimmediately after carburization and melting obtained by a manufacturingexperiment.

FIG. 10 is an experimental graph demonstrating that the sulfur contentof the metallic iron nuggets can be decreased by adjusting the basicityof the slag by intentionally adding a CaO source to material compacts.

FIG. 11 is a graph showing the relationship between the sulfur contentof the metallic iron nuggets and the basicity of the product slag.

FIG. 12 is an explanatory diagram showing the composition of thematerial, and the ratio and the composition of the products such asmetallic iron nuggets produced by a manufacturing process employed inExample.

FIG. 13 is a photograph of metallic iron nuggets prepared in Example 1.

FIG. 14 is an explanatory diagram showing the composition of thematerial, and the ratio and the composition of the products such asmetallic iron nuggets produced by a manufacturing process employed inanother Example.

FIG. 15 is a photograph of metallic iron nuggets prepared in Example 2.

FIG. 16 is a graph showing the relationship between the diameter of thematerial compacts (dry pellets) and an average diameter and an averagemass of the produced metallic iron nuggets.

BEST MODE FOR CARRYING OUT THE INVENTION

Metallic iron nuggets of the invention are granular metallic iron madeby reducing-melt of a material containing a carbonaceous reductant andan iron-oxide containing material. The metallic iron nuggets contain 94%or more (more preferably 96% or more) of Fe and 1.0 to 4.5% (morepreferably 2.0 to 4.0%) of C. Preferably, the S content of the metalliciron nuggets is 0.20% or less, more preferably, 0.08% or less, and thediameter is in the range of 1 to 30 mm (more preferably 3 to 20 mm). Thereasons for setting these ranges are as follows.

The Fe content of the metallic iron nuggets is the primary factor thatcontrols the quality of the metallic iron nuggets. Naturally, the higherthe Fe purity, i.e., the lower the contaminant content, the better. Inthe present invention, the required Fe purity is 94% or more, and morepreferably, 96% or more. The reason for this is as follows. Whenmetallic iron nuggets having a contaminant content exceeding 5% are usedas a material for iron and steelmaking, the contaminants contained inthe material float on the surface of a bath and form slag, which isdifficult to remove. Moreover, because elements, such as S, Mn, Si, andP, dissolved in a molten steel adversely affect the physical propertiesof the end products made using the resulting metallic iron, processessuch as desulfurization, dephophorization, and desiliconization arenecessary during an refinning step. These preliminary treatments requiresubstantial time and effort. Accordingly, the Fe content of the metalliciron nuggets of the present invention must be at least 94%, and morepreferably, at least 96%.

The C content of the metallic iron nuggets is essential in securing therequired amount of C to suit the steel grade when the metallic iron isused as a material for steelmaking, and is important in view ofincreasing versatility as material iron. Accordingly, the C content ofthe metallic iron nuggets is preferably at least 1.0%, and morepreferably, at least 2.0%. When the metallic iron contains excessiveamounts of carbon, the tenacity and the shock resistance of steel oralloy steel made from such metallic iron are adversely affected, andthus the steel or alloy steel becomes fragile. Thus, a decarburizationprocess such as blowing becomes necessary during the process ofrefinning. In order to use the metallic iron nuggets as a material foriron and steelmaking without being burdened by these additionalprocesses and without hindrance, the C content must be 4.5% or less, andmore preferably, 4.0% or less.

Sulfur adversely affects the physical properties of steel and is thususually considered undesirable, although sulfur can be used to increasemachinability of some types of steel grade. The metallic iron nuggets ofthe invention used as a material preferably contain 0.20% or less, andmore preferably, 0.08% or less of sulfur. In order to increase theapplicable range of the metallic iron nuggets as an iron source so thatthe metallic iron nuggets can be used in various steelmaking processes,the Si content should be in the range of 0.02 to 0.5%, and the Mncontent should be less than 0.3%.

The metallic iron nuggets of the invention having the above-described C,S, Si, and Mn contents are particularly advantageous when compared tomost commonly used pig iron made using blast furnaces. The pig iron madeusing blast furnaces generally contains 4.3 to 4.8% C, 0.2 to 0.6% Si,and 0.3 to 0.6% Mn, although the contents of C, S, Mn, Si, and the likein the pig iron made using a blast furnace vary according to the type ofmetal oxide and coke used therein, operation conditions, and the like.Especially in blast furnace iron making, the produced molten metalliciron is carburized at the bottom part of the blast furnace in a highreducing atmosphere in the presence of a large amount of coke; hence,the C content is nearly saturated. Since SiO₂, which is included as agangue component, is readily reduced in a high-temperature atmosphere inthe presence of a large amount of coke, approximately 0.2 to 0.6% of Siis contained in the molten metallic iron, and it is difficult to obtainmolten metallic iron having a Si content of less than 0.20%. Moreover,since MnO is easier to reduce than SiO₂, MnO is readily reduced in ahighly reducing atmosphere when a large amount of MnO is included in thematerial iron ore. As a result, the Mn content in the molten metalliciron becomes inevitably high.

In contrast, the metallic iron nuggets of the present invention made bya process described below contain 1.0 to 4.5% C, 0.02 to 0.5%, and morepreferably less than 0.2%, Si, and less than 0.3% Mn. The metallic ironnuggets of the present invention differ from common metallic irondescribed above in the composition. Furthermore, as described below, theS content of the metallic iron nuggets of the present invention isreduced by using a CaO source during the step of making a materialcompact so as to increase the basicity of the slag components. Themetallic iron nuggets of the present invention is distinguishable frommetallic iron made according to a common process in that the S contentis 0.08% or less.

It is essential that the metallic iron nuggets of the present inventionhave a diameter in the range of 1 to 30 mm. Minute particles having adiameter less than 1 mm cause quality and handling problems because fineslag components easily become mixed with such minute particles and suchminute particles of metallic iron fly off easily.

The upper limit of the diameter is set in view of reliably obtaining apredetermined level of the Fe purity within required manufacturingrestrictions. In order to obtain large nuggets having a diameterexceeding 30 mm, large compacts must be used as a material. With suchlarge material compacts, the time taken to conduct heat toward theinside of the material compacts during a process of solid reduction,carburization, and melting, particularly during solid reduction, formaking metallic iron nuggets, is long, decreasing the efficiency ofsolid reduction. Moreover, the incorporation of the molten iron aftercarburization and melting due to cohesion does not proceed uniformly. Asa result, the produced metallic iron nuggets have complex and irregularshapes, and metallic iron nuggets having a uniform diameter and qualitycannot be obtained.

The size and shape of the iron nuggets are affected by various factorsincluding the size of the material compacts as described above, thecomposition of the material (the type of metal oxide source and thecomposition of the slag), the carburization amount after solidreduction, the furnace atmosphere temperature (particularly theatmosphere temperature in the region where carburization, melting, andcohesion are performed), and the supply density at which the materialcompacts are supplied to the reducing-melt furnace. The supply densityand the size of the material compacts have the same influence. Thehigher the supply density, the likelier it is for the molten metalliciron produced by carburization and melting to form large nuggets on ahearth due to cohesion and incorporation. By gradually increasing thesupply density of the material compacts and eventually stacking thematerial compacts on a hearth, the chance that molten metallic ironincorporates to form large nuggets can be increased. However, when thesupply density is excessively high, the heat conduction ratio in thefurnace decreases, and thus the solid reduction ratio cannot beincreased. Moreover, uniform cohesion and incorporation becomedifficult, and the resulting metallic iron nuggets will have complex andirregular shapes. Metallic iron nuggets having a uniform diameter and auniform shape cannot be obtained.

These problems derived from the size of the material compacts and thelike are particularly acute when metallic iron nuggets having a diameterof 30 mm or more as products are made. No such problems occur in makingnuggets having a diameter of 30 mm or less, and nuggets having arelatively uniform diameter of 30 mm or less and a relatively uniformshape can be obtained. In view of the above, the diameter is limited to30 mm or less in the present invention. It should be noted that nuggetshaving a highly uniform diameter, shape, and quality can be obtained ata diameter of 3 to 15 mm.

The size of the produced metallic iron nuggets is also affected by thetype and the characteristics of the iron ore contained in the materialcompacts. Generally, the cohesion property is satisfactory whenmagnetite iron ore is used as an iron oxide source. However, not all ofthe iron content in one material compact necessarily coheres into onemetallic iron nugget. The iron content in one material compactfrequently forms two or three nuggets. The cause of such a phenomenon isnot precisely known, but a complex combination of difference in oxygencontent, in crystal structure of iron ore, in slag composition derivedfrom the gangue composition are considered as possible causes. In anycase, metallic iron nuggets having a relatively uniform diameter andshape can be obtained at a diameter of the product nuggets of 30 mm orless.

The metallic iron nuggets of the present invention satisfy all of therequirements described above and can be effectively used as an ironsource for making iron, steel, or alloy steel using various facilitiesfor iron, steel, or alloy-steelmaking, such as an electric furnace.

An embodiment of a method for making metallic iron nuggets satisfyingthe above requirements will now be described in detail with reference tothe drawings.

FIGS. 1 to 3 are schematic illustrations showing an example of areducing-melt furnace of a rotary hearth type developed by the inventorsused for making metallic iron nuggets of the present invention. Thereducing-melt furnace has a ring-shaped movable hearth and a dome-shapedstructure. FIG. 1 is a schematic illustration thereof, FIG. 2 is across-sectional view taken along line A-A in FIG. 1, and FIG. 3 is across-sectional view of the movable hearth, developed in a movingdirection to promote understanding of the structure. In the drawings,reference numeral 1 denotes a rotary hearth, and reference numeral 2denotes a furnace casing that covers the rotary hearth. The rotaryhearth 1 is configured to rotate at an adequate speed by a driver notshown in the drawing.

A plurality of combustion burners 3 is provided at suitable positions ofthe wall of the furnace casing 2. The combustion heat and the radiantheat thereof from the combustion burners 3 are applied to materialcompacts on the rotary hearth 1 so as to perform heat reduction of thecompacts. The furnace casing 2 shown in the drawing is a preferableexample and is divided by three partitions K₁, K₂, and K₃ into a firstzone Z₁, a second zone Z₂, a third zone Z₃, and a fourth zone Z₄. At theuppermost stream in the rotation direction of the furnace casing 2, afeeder 4 for feeding material and an auxiliary material, the feeder 4facing the rotary hearth 1, is provided. At the lowermost stream in therotation direction, i.e., the position upstream of the feeder 4 becauseof the rotatable structure, a discharger 6 is provided.

In operating this reducing-melt furnace, while allowing the rotaryhearth 1 to rotate at a predetermined speed, material compactscontaining iron ore or the like and a carbonaceous substance aresupplied from the feeder 4 until an adequate thickness is reached. Thematerial compacts placed on the rotary hearth 1 receive the combustionheat and the radiant heat thereof from the combustion burners 3 duringthe course of traveling through the first zone Z₁. The metal oxidecontained in the compacts is reduced while sustaining its solid statedue to the carbonaceous substance in the compacts and carbon monoxideproduced by burning the carbonaceous substance. Subsequently, thematerial compacts are further reduced by heating in the second zone Z₂.The resulting iron, which is substantially completely reduced, is thenfurther heated in a reducing atmosphere in the third zone Z₃ so as tocarburize and melt the reduced iron while allowing the reduced iron toseparate from by-product slag and form nuggets, i.e., metallic ironnuggets. Subsequently, the resulting metallic iron nuggets are cooledand solidified in the fourth zone Z₄ by a suitable cooling means C, andare sequentially discharged by the discharger 6 at the downstream of thecooling means C. At this time, the by-product slag derived from thegangue component, etc., in the iron ore is also discharged. Theby-product slag is separated from the metallic iron by suitableseparating means, such as a screen and a magnetic separation apparatus,after the slag and the metallic iron is fed to a hopper H. The resultingmetallic iron nuggets have an iron purity of approximately 94% or more,and more preferably, 96% or more, and contain a significantly low amountof the slag component.

It should be noted that although the fourth zone Z₄ in the drawing is ofan open-air type, the fourth zone Z₄ is preferably provided with a coverso as to prevent heat dissipation as much as possible and to suitablyadjust the atmosphere inside the furnace in actual operation. Moreover,although, in this embodiment, the rotary furnace is divided into thefirst zone Z₁, the second zone Z₂, the third zone Z₃, and the fourthzone Z₄ using three partitions K₁ to K₃, the zone configuration of thefurnace is not limited to this structure. Naturally, the zoneconfiguration may be modified according to the size of the furnace, therequired manufacturing capacity, the operation mode, or the like.However, in order to efficiently manufacture the metallic iron nuggetsof the present invention, a structure in which a partition is providedat least between the solid-reduction area of the first half period ofthe heating reduction, and the carburization, melting, and cohesion areaof the second half period of the heating reduction so that the furnacetemperature and the atmosphere gas can be separately controlled ispreferable.

During the above reducing-melt process, when the atmosphere temperatureduring the reduction (solid reduction period) is excessively high, i.e.,when the atmosphere temperature exceeds the melting point of the slagcomponents including the gangue component, unreduced iron oxide, and thelike during a certain period in the reduction process, iron oxide (FeO)in the material melts before it is reduced. As a result,smelting-reduction rapidly occurs due to the reaction of the molten ironoxide with carbon contained in the carbonaceous substance. Note thatsmelting-reduction is a phenomenon in which a material is reduced in amolten state, and is different from solid reduction. Metallic iron canstill be produced by smelting-reduction; however, when reduction occursin the molten state, the separation of reduced iron from by-product slagis difficult. Moreover, the reduced iron is obtained in the form of asponge, which is difficult to make nuggets therefrom, and the slagcontent in the reduced iron becomes high. Accordingly, it becomesdifficult to achieve an Fe content within the range specified by thepresent invention. Furthermore, the molten metallic iron formed byincorporation due to cohesion may flow on the hearth and may becomeplanular instead of granular.

FIG. 4 shows the state of the reaction when material compacts (pelletshaving a diameter of 16 to 19 mm) containing iron ore as an iron oxidesource and coal as a carbonaceous reductant are fed to a furnace havingan atmosphere temperature of approximately 1,300° C. (the straight line1 in the graph) so as to solid-reduce the material compacts until areduction ratio of 100% (the elimination ratio of oxygen in the ironoxide in the material compacts) is reached, and then the resultingreduced iron is fed to a melting zone controlled at approximately 1425°C. (straight line 2) beginning at the time indicated by straight line 3in the drawing so as to melt the resulting reduced iron. In the graph,the temperature inside the compacts, the atmosphere temperature of thefurnace, and changes of carbon dioxide and carbon monoxide over timeproduced during the reduction process are also shown. The temperatureinside the compacts is continuously measured using a thermocoupleinserted into the material compacts in advance.

As is apparent from this graph, in order to maintain the solid state ofthe material compacts fed into the furnace and to reduce the materialcompacts to a reduction ratio (oxygen elimination ratio) of 80% (point Ain FIG. 4) or more, and more preferably, 94% (point B in FIG. 4) ormore, the furnace temperature is preferably maintained in the range of1,200 to 1,500° C., and more preferably, 1,200 to 1,400° C., to performsolid reduction, and subsequently increased to 1,350 to 1,500° C. toreduce the remaining iron oxide while allowing the produced metalliciron to form nuggets by carburization and melting. According to thistwo-stage heating process, metallic iron nuggets having a high Fe puritycan be reliably and efficiently manufactured.

The time indicated by the horizontal axis in FIG. 4 may vary dependingon the composition of the iron ore or the carbonaceous substanceconstituting the material compacts. Normally, solid reduction of theiron oxide, melting, cohesion, and incorporation can be completed andmetallic iron nuggets can be made within 10 to 13 minutes.

If solid reduction of the material compacts is stopped at a reductionratio of less than 80% and melting is started therefrom, sponge-shapedmetallic iron is produced, and formation of nuggets from such metalliciron is difficult. Moreover, it is difficult to achieve an Fe content of94% or more in the resulting metallic iron. In contrast, when the solidreduction is performed until a reduction ratio of 80% or more, and morepreferably 94% or more is reached and then the subsequent step ofcarburization, melting, and cohesion is performed, the remaining FeO inthe material compacts can be effectively reduced regardless of the typeand the composition of the iron ore in the material compacts. Moreover,in the subsequent step of carburization and melting, nuggets can growwhile excluding the by-product slag. Thus, metallic iron nuggets havinga high Fe content and a relatively uniform diameter can be obtained.

In the solid-reduction region shown in the first part of FIG. 4, thepreferable furnace temperature that can securely achieve a highreduction ratio is 1,200 to 1,500° C., and more preferably 1,200 to1,400° C. At a furnace temperature of less than 1,200° C., the solidreduction reaction progresses slowly, and thus the dwell time in thefurnace must be made longer, resulting in poor productivity. At afurnace temperature of 1,200° C. or more, and particularly 1,500° C. ormore, the metallic iron nuggets incorporate with one another to formlarge nuggets of irregular shapes. Such metallic iron nuggets are notpreferable as a product.

The metallic iron nuggets may not incorporate with one another to formlarge nuggets in a temperature range of 1,400 to 1,500° C. depending onthe composition and the amount of the iron ore in the material. However,this possibility and frequency are low. Thus, the temperature during thesolid reduction period is preferably 1,200 to 1,500° C., and morepreferably 1,200 to 1,400° C. In actual operation, it is possible toadjust the furnace temperature to 1,200° C. during the early stage ofthe solid reduction period and then increase the furnace temperature to1,200 to 1,500° C. during the latter stage of the solid reduction.

The compacts subjected to the required reduction in the solid-reductionzone are transferred to a melting zone having a high furnace temperatureof 1,425° C. The temperature inside the compacts increases as shown inFIG. 4, drops after reaching a point C, and then increases again until apredetermined temperature of 1,425° C. is reached. The temperature dropat point C is caused by latent heat accompanying melting of the reducediron, i.e., the point C can be considered as the starting point of themelting. This starting point is substantially determined by the residualcarbon content in the reduced iron particles. Since the melting point ofthe reduced iron drops as a result of the carburization by the residualcarbon and a CO gas, melting of the reduced iron is accelerated.

In order to rapidly melt the reduced iron, a sufficient amount of carbonfor carburization must remain in the reduced iron after the solidreduction. The content of the residual carbon is determined by theamount of the iron ore and the carbonaceous substance used in making thematerial compacts. The inventors have confirmed through experiments thatwhen the amount of the carbonaceous substance is initially adjusted sothat the residual carbon content, i.e., the excess carbon content, inthe solid-reduced substance is 1.5% at the time the final reductionratio during the solid-reduction period reaches 100%, i.e., at the timethe metallization ratio reaches 100%, the reduced iron can be rapidlycarburized, thereby causing a drop in the melting point. Accordingly,the reduced iron can rapidly form nuggets having a suitable diameter bycohesion and incorporation in a temperature range of 1,300 to 1,500° C.Note that when the residual carbon content of the solid-reduced carbonis less than 1.5%, the melting point of the reduced iron does not dropsufficiently due to the shortage of carbon for carburization, and theheating temperature must thus be increased to 1,500° C. or more.

When the carburization amount is zero, i.e., when pure iron is involved,the melting temperature is 1,530° C., and the reduced iron can be meltedby heating at a temperature exceeding this temperature. However, inactual furnaces, the operating temperature is preferably low to reduceheat load imposed on furnace refractories. The operating temperature ispreferably approximately 1,500° C. or less. In particular, the operatingconditions are preferably adjusted to allow a temperature increase ofapproximately 50 to 200° C. after the staring point C of melting, whichis the beginning of the melting and cohesion period. In order tosmoothly and effectively perform the above-described solid reduction,carburization, and melting, the temperature during the carburization andmelting is preferably 50 to 200° C., and more preferably, 50 to 150° C.,higher than the temperature during the solid reduction.

In this invention, the final carbon content in the end product metalliciron nuggets must be in the range of 1.0 to 4.5%, and more preferably,2.0 to 4.0%. The final carbon content is substantially determined by theamount of the carbonaceous substance used in making material compactsand atmospheric adjustments during the solid-reduction period.Especially, the lower limit of the carbon content is determined by theresidual carbon content in the reduced iron during the final stage ofthe solid reduction and the retention time (carburization amount) duringthe period following the period of solid reduction. If a reduction ratioof 100% is nearly achieved during the final stage of the solid reductionas described above while securing 1.5% of the residual carbon content,the end product of the metallic iron nuggets can have a carbon contentof 1.0% or more. Moreover, the inventors have also confirmed that whenthe residual carbon content in the reduced iron upon completion of thesolid reduction is 5.0% and carburization, melting, and cohesion of thisreduced iron are performed during the subsequent period of melting andcohesion, the carbon content in the resulting metallic iron nuggets canbe increased to 4.5%. However, in order to reliably obtain metallic ironnuggets having a final carbon content of 2.0 to 4.0%, the residualcarbon content in the reduced iron after completion of the solidreduction is preferably controlled in the range of 1.5 to 4.5%.

As for the atmosphere gas in the process, during the period in whichsolid reduction is rapidly progressed, a large amount of CO is generatedby the reaction of the metal oxide with the carbonaceous substance inthe material compacts, and the region adjacent to the compacts ismaintained at a high reducing atmosphere due to the self-shieldingeffect. However, during the latter stage of the solid reduction andduring the subsequent carburization and melting period, the amount ofthe CO gas produced drastically decreases. Thus, prevention ofreoxidation due to the self-shielding effect cannot be expected.

FIG. 5 shows results of examination on the relationship among themetallization ratio, the residual FeO, and the residual carbon in theresulting material of the solid reduction. As shown in the graph, FeOdecreases as solid reduction progresses, that is, as the metallizationratio increases. Up to straight line 1 in the graph, solid reduction ofthe material compacts progresses inside the furnace controlled at atemperature of 1,200 to 1,500° C. Subsequently, carburization, melting,and cohesion of the reduced iron progress during the melting period inwhich the temperature is controlled in the range of 1,350 to 1,500° C.in a highly reducing atmosphere. During this period, the relationshipamong the metallization ratio, the residual FeO and the residual carbonchanges as shown by the portions of the curves included in the rightsection of the graph from the straight line 1.

Curves (1) and (2) in FIG. 5 show the relationship between themetallization ratio and the residual carbon content. The curve (1) iswhen the residual carbon content is 1.5% when the metallization ratio is100%. The curve (2) is when the residual carbon content is 3.0% when themetallization ratio is 100%. In order to obtain the metallic ironnuggets of the present invention, the amount of the carbonaceoussubstance is preferably controlled during the process of making materialcompacts so that the residual carbon content is above the curve (1).

Note that even when a predetermined amount of the carbonaceous substanceis used in making material compacts, the residual carbon content at themetallization ratio of 100% slightly varies depending on the reducingdegree of the atmosphere gas inside the furnace. Accordingly, the amountof the carbonaceous substance should be suitably adjusted according tothe reducing degree of the atmosphere gas used in the operation. In anycase, the initial amount of the carbonaceous substance is preferablyadjusted so that the final residual carbon content is 1.5% or more at ametallization ratio of 100%.

FIG. 6 shows the results of the examination on the relationship betweenthe residual carbon content at a metallization ratio of 100% and the Ccontent of the resulting metallic iron nuggets. When the residual carboncontent is 1.5 to 5.0%, the resulting metallic iron nuggets can securelyhave a C content of 1.0 to 4.5%. When the residual carbon content is 2.5to 4.5%, the resulting metallic iron nuggets can securely have a Ccontent of 2.0 to 4.0%.

In the description above, two indices, i.e., the metallization ratio andthe reduction ratio, are used to indicate the state of FeO reduction.The definitions of the metallization ratio and the reduction ratio aredescribed below. The relationship between the two is, for example, shownin FIG. 7. The relationship between the two changes depending on thetype of the iron ore used as an iron oxide source. FIG. 7 shows therelationship between two when magnetite (Fe₃O₄) is used as an iron oxidesource.Metallization ratio=[metallic iron nuggets produced/(metallic ironnuggets produced+iron in iron ore)]×100(%)Reduction ratio=[amount of oxygen removed during the reductionprocess/amount of oxygen in the iron oxide contained in the materialcompacts]×100(%)

The reducing-melt furnace used in making the metallic iron nuggets ofthe present invention employs burners to heat the material compacts, asdescribed above. During the solid-reduction period, as described abovewith reference to FIG. 4, the iron oxide source and the carbonaceoussubstance in the material compacts fed into the furnace react with eachother to produce a large amount of CO gas and a small amount of CO₂ gas.Accordingly, the region adjacent to the material compacts is maintainedat a sufficient reducing atmosphere as a result of the shielding effectof the CO gas emitted from the material compacts themselves.

However, during the latter stage and the final stage of the solidreduction period, the amount of the CO gas decreases rapidly, resultingin a decrease in the self-shielding effect. Accordingly, the reducediron becomes vulnerable to the exhaust gas, i.e., an oxidizing gas suchas CO₂ and H₂O, produced by burner heating, and reoxidation of thereduced metallic iron may occur. Moreover, after completion of the solidreduction, melting and cohesion of the minute particles of reduced ironprogress due to the carburization of the reduced iron using the residualcarbon in the compacts and a decrease in the melting temperatureresulting from the carburization. During this stage also, since theself-shielding effect is poor, the reoxidation of the reduced iron mayreadily occur.

In order to efficiently perform carburization, melting, and cohesionafter the solid reduction to secure an Fe purity of 94% or more and tothereby obtain metallic iron nuggets of a suitable diameter whilepreventing a decrease in the Fe purity resulting from such reoxidationas much as is feasibly possible, the composition of the atmosphere gasin the carburization and melting regions is preferably optimized.

In view of the above, the examination of atmosphere conditions forefficiently performing carburization and melting while preventing thereoxidation of the reduced iron during the carburization and meltingperiod after completion of the solid reduction was conducted. Theresults of the examination will now be described with reference to FIG.8. In the experiments, a box furnace was used, and coal powder was usedas an atmosphere adjustor during the carburization and melting stage. Ona hearth, a coal powder was bed to an adequate thickness so as to keep ahighly reducing atmosphere during the carburization and melting.

In particular, coal powders having different particle diameters wereused as atmosphere adjustors. The coal powder was bedded to a thicknessof approximately 3 mm on an alumina tray, and 50 to 60 material compactshaving a diameter of approximately 19 mm were placed on the bed of thecoal powder. A thermocouple was provided to one of the materialcompacts. The material compacts were fed into the box furnace. Thetemperature of the composite during heating was measured, and thecomposition of the gas produced was measured to determine thepossibility of the reoxidation of the produced metallic iron. Note thatthe temperature inside the electric furnace was adjusted so that themaximum furnace temperature is approximately 1,450° C. The initialcomposition of the atmosphere gas inside the furnace was CO₂: 20% andN₂: 80%.

FIG. 8 shows the results of the experiments in which the temperature ofthe material compacts detected by the thermocouple described above andthe composition of the atmosphere gas when the temperature inside theelectric furnace is gradually elevated were measured over time. Thehorizontal axis shows changes in temperature, and the vertical axisshows a simplified reducing degree (CO)/(CO+CO₂) of the atmosphere gas.In the graph, four experimental results are plotted. Curve (3) shows theresult of the experiment where no atmosphere adjustor was used. Curve(4) shows the result of the experiment where a coarse coal powder havingan average diameter exceeding 3.0 mm was used as an atmosphere adjustor.Curves (1) and (2) show the results of the experiments where fine coalpowders A and B having a diameter of 2.0 mm or less were used. In thegraph, an FeO—Fe equilibrium curve and an Fe₃O₄—FeO equilibrium curveare also included. The circled regions indicate periods during which thesolid reduction nearly completes and the carburization, melting, andcohesion of the reduced iron begin in these experiments. The control ofthe atmosphere gas during these periods is particularly important forpreventing reoxidation of the iron oxide and for obtaining metallic ironnuggets of a high Fe purity.

As is apparent from this graph, the curve (3) of the experiment where noatmosphere adjustor was used, the region C indicating the beginning ofthe carburization, melting, and cohesion of the reduced iron, was farbelow the FeO—Fe equilibrium curve. This demonstrates that the entirereduced iron melted while a portion thereof underwent the reducing-melt.The metallic iron was still obtained in this experiment, but, asdescribed above, when reducing-melt occurs, the resulting iron is likelyto be sponge-shaped and is thus easy to make nuggets therefrom.Moreover, the Fe purity of the metallic iron was insufficient.

In contrast, the curves (1) and (2) show the results of the experimentsin which fine coal powder was used. As is apparent from the graph, thereducing degree of the atmosphere gas was significantly improved.Moreover, the region A in which the carburization, melting, and cohesionof the reduced iron occurred was above the FeO—Fe equilibrium curve,meaning that the generation of FeO was prevented in these experiments.The curve (3) shows the results of the experiment using a coarse coalpowder. In this experiment, the region B in which the carburization,melting, and cohesion of the reduced iron occurred was slightly belowthe FeO—Fe equilibrium curve. This means some degree of reoxidationmight have occurred. However, the composition of the produced metalliciron was examined, and the results confirmed that substantially noreoxidation occurred in this experiment.

It was also confirmed that the metallic iron nuggets having an Fecontent of 94% or more and a carbon content of 1.0 to 4.5% can be highlyeffectively manufactured by controlling the reducing degree of theatmosphere gas to at least 0.5, more preferably, at least 0.6, yet morepreferably, at least 0.7, and most preferably above the FeO—Feequilibrium curve, at least during the beginning stage of thecarburization, melting, and cohesion period. In this manner,carburization, melting, and cohesion can be smoothly performed withoutallowing the reoxidation of the reduced iron produced by solidreduction.

Direct analysis of the experimental data shown in FIG. 8 suggests that asubstantial degree of reoxidation may occur at a simplified reducingdegree of 0.5 to 0.7. However, this experiment examines the reoxidationdegree of the atmosphere gas only; the inner portions of the actualmaterial compacts or the atmosphere near the actual material compactsare maintained at a highly reducing atmosphere because of the presenceof the residual carbon inside the material compacts and the atmosphereadjustor. Moreover, an oxidizing gas such as CO₂ and H₂O in theatmosphere of the upper portion of the hearth is readily reduced by thecarbonaceous atmosphere adjustor when the oxidizing gas enters thesection near the material compacts. Thus, it is assumed that noreoxidation occurs even when the measured reducing degree of theatmosphere is 0.5 to 0.7. Note that at a reducing degree of less than0.5, the produced metallic iron is readily reoxidized, cohesion of themetallic iron and formation of metallic iron nuggets become difficultdue to insufficient carburization, and metallic iron nuggets having adiameter in the range of the present invention are difficult to obtain.

After carburization, melting, and cohesion of the reduced iron arecompleted, the reducing degree of the atmosphere gas decreases rapidly.However, in actual operation, the metallic iron, which has been meltedand cohered, is nearly completely separated from the by-product slag bythis time. Thus, the metallic iron is hardly affected by the atmospheregas, and metallic iron nuggets having a high Fe content and a lowinclusion slag content can be effectively made by cooling andsolidifying this metallic iron.

As is apparent from above, a coal powder used as an atmosphere adjustoris preferably pulverized to a diameter of 3 mm or less, and morepreferably, 2 mm or less to further reliably prevent the reoxidationduring carburization, melting, and cohesion. In view of the yield andoperation of the furnace in actual operation, the diameter of the coalpowder is most preferably in the range of 0.3 to 1.5 mm. No limit isimposed as to the thickness at which the coal powder is bedded, but thethickness of the coal powder bed is preferably approximately 2 mm ormore, and more preferably 3 mm or more since the amount of the coalpowder as the atmosphere adjustor is insufficient at an excessive smallthickness. No limit is imposed as to the upper limit of the thickness.However, since the atmosphere adjusting effect saturates at anexcessively large thickness, it is practical and cost-effective torestrict the thickness to preferably approximately 7 mm or less, andmore preferably, approximately 6 mm or less. Any material can be used asan atmosphere adjustor as long as it releases CO. Examples of suchmaterials include coal, coke, and charcoal. These materials may be usedalone or in combination.

The atmosphere adjustor may be bedded on a hearth before the materialcompacts are fed on a hearth. In such a case, the atmosphere adjustoralso functions to protect the hearth refractory from the slag bleedingduring the reducing-melt process. However, since the atmosphere adjustorexerts its effect during the carburization, melting, and cohesion periodafter the solid reduction, it is also effective to sprinkle theatmosphere adjustor from above the hearth immediately before thecarburization and melting of the material compacts begin.

According to the above method, the reoxidation of the reduced iron canbe prevented and carburization, melting, and formation of nuggets can beeffectively performed since the reducing degree of the atmosphere gasduring the carburization and melting period is enhanced. Thus, metalliciron nuggets having a high Fe content and a suitable size can beefficiently manufactured. During the process, in order to effectivelyperform a series of steps from solid reduction to the carburization,melting, and cohesion, the temperature and the atmosphere gas arepreferably separately controlled according to the step. In particular,the temperature during the solid reduction period is preferably 1,200 to1,400° C. to prevent reducing-melt reaction, as described above. Thetemperature during the carburization, melting, and cohesion period ispreferably 1,300 to 1,500° C. More preferably, the temperature duringthe solid reduction period is 50 to 200° C. lower than the temperatureduring the carburization, melting, and cohesion period.

As for the atmosphere gas conditions, since a large amount of CO gasthat is produced by the burning of the carbonaceous substance inside thematerial compacts maintains a highly reducing atmosphere during thesolid reduction period, the atmosphere gas inside the furnace does notrequire extensive control. In contrast, during the carburization,melting, and cohesion period, emission of the CO gas from the materialcompacts drastically decreases. As a result, reoxidation caused by theoxidizing gas produced by the combustion of the burners may readilyoccur. Thus, in order to obtain metallic iron nuggets having an adequatecarbon content, it is essential to suitably adjust the atmosphere gasinside the furnace from this period on. The atmosphere gas can beadjusted by using an atmosphere adjustor, for example.

In order to suitably adjust the temperature and the atmosphere gascomposition inside the furnace according to the progress of thereducing-melt, the reducing-melt furnace is preferably divided into atleast two zones in the traveling direction of the hearth by using apartition, as shown in FIGS. 1-3. Preferably, the upstream zone isconfigured as a solid reduction zone, and the downstream zone isconfigured as a carburization, melting, and cohesion zone so as toseparately control the temperature and the atmosphere gas composition ofeach zone. Note that FIG. 3 shows as example in which the furnace isdivided into four zones using three partitions to allow more stringentcontrol of the temperature and the atmosphere gas composition. Thenumber of zones can be adjusted to suit the scale and the structure ofthe reducing-melt facility.

The metallic iron nuggets of the present invention made by theabove-described process contain substantially no slag component and havean Fe purity of 94% or more, and more preferably 96% or more, and acarbon content of 1.0 to 4.5%. The diameter thereof is in the range of 1to 30 mm. These metallic iron nuggets are used as an iron source inknown facilities for steelmaking, such as a electric furnace and aconverter. When using the metallic iron nuggets as a material forsteelmaking, the sulfur content therein is preferably as low as isfeasibly possible. The investigation has been conducted to remove sulfurcontained in the iron ore and the carbonaceous substance as much aspossible during the process of making the metallic iron nuggets and toobtain metallic iron nuggets having a low sulfur content.

As a result, it has been found that the sulfur content in theend-product metallic iron nuggets can be reduced to 0.08% or less byintentionally adding a CaO source, e.g., burnt lime, slaked lime, orcalcium carbonate, during making the material compacts using the ironore and the carbonaceous substance so as to adjust the basicity (i.e.,the ratio of CaO/SiO₂) of the overall slag components contained in thematerial compacts to 0.6 to 1.8, and more preferably 0.9 to 1.5, theoverall slag components including the gangue component in the iron ore,etc.

Note that coke or coal, which is the most commonly used carbonaceousreductant, normally contains approximately 0.2 to 1.0% of sulfur. Themajority of sulfur contained therein is captured in the metallic iron.If basicity adjustment intentionally using a CaO source is notperformed, the basicity calculated based on the slag composition in thematerial compacts is usually 0.3 or less, although the basicitysignificantly varies according to the type of iron ore. In slag havingsuch a low basicity, sulfur cannot be prevented from becoming mixed intothe metallic iron during the solid reduction process or the subsequentprocess of carburization, melting, and cohesion. Approximately 85% oftotal sulfur in the material compacts will be included in the metalliciron. As a result, the sulfur content of the metallic iron nuggets isincreased, and the quality of the end-product metallic iron is degraded.

It was confirmed that by intentionally adding a CaO source during thestep of making material compacts so as to adjust the composition of theslag component to exhibit a basicity of 0.6 to 1.8, sulfur can be fixedin the by-product slag which is produced during solid reduction andcarburization, melting, and cohesion. As a result, the sulfur content inthe metallic iron nuggets can be dramatically reduced.

The sulfur content reduction is considered to occur when sulfurcontained in the material compacts is allow to react with CaO and isthus fixed as CaS (CaO+S=CaS). Conventionally, when the above-describedreducing-melt mechanism was not clearly known, it was considered thatdesulfurization effect comparable to that of a hot metal desulfurizationcannot be achieved by the addition of CaO. However, the inventors haveconfirmed that CaO in the slag captures sulfur when the reduced ironmelts, forms nuggets, and becomes separated from the slag due to thecarburization caused by the residual carbon inside the reduced metal,and thus the sulfur content in the resulting metallic iron nuggets canbe dramatically decreased.

Such a sulfur reduction mechanism is different from a normal hot metaldesulfurization using CaO-containing slag and is considered as areaction unique to the above-described process. Of course, if carburizedand melted reduced iron is sufficiently put into contact with theby-product molten slag under appropriate heating conditions, aliquid-liquid (molten iron-molten slag) reaction may determine the ratioof the S content in the slag (S %) to the S content in the metallic ironnuggets [S %], i.e., the distribution ratio of sulfur (S %)/[S %].However, as can be confirmed by the photograph shown in FIG. 9, theslag-metal contact area of the produced molten iron and the molten slagis small. Thus, a large sulfur reduction cannot be expected from theslag-metal equilibrium reaction after the reduced iron is carburized,melted, and cohered. Accordingly, it can be assumed that thedesulfurization mechanism of intentionally adding CaO into the materialcompacts employed in the above process includes a sulfur trappingreaction peculiar to CaO during carburization, melting, and cohesion ofreduced iron, the sulfur trapping reaction preventing the sulfurizationof the metallic iron nuggets.

The amount of the CaO added to adjust the basicity should be determinedbased on the amount and the composition of the gangue componentcontained in iron ore or the like and on the type and the amount of thecarbonaceous substance added to the material. A standard amount of CaOrequired to adjust the basicity of the overall slag component in theabove-described range of 0.6 to 1.8 is, in terms of pure CaO, 2.0 to7.0%, and more preferably 3.0 to 5.0%, of CaO in the entirety of thecomposites. When slaked lime [Ca(OH)₂] or calcium carbonate (CaCO₃) isused, the amount thereof should be converted to CaO. It was confirmedthat when 4% CaCO₃ was contained in the material compacts to adjust thebasicity of the slag component to approximately 0.9 to 1.1, an apparentdesulfurization ratio of 45 to 50% was obtained. The apparentdesulfurization ratio was determined by the equation below. When 6%CaCO₃ was contained in the material compacts to adjust the basicity ofthe slag component to approximately 1.2 to 1.5, an apparentdesulfurization ratio of 70 to 80% was obtained.

Apparent desulfurization ratio (%)=[S content (%) in the metallic ironnuggets made from CaO-added material compacts/S content (%) in themetallic iron nuggets made from material compacts not using an additiveCaO]×100.

The effect of adding a CaO source to the material on reduction of sulfurwill now be described based on experimental data taken using a boxfurnace. FIG. 10 shows changes in sulfur content when reducing-melt isperformed as described above using iron ore, a carbonaceous substance, asmall amount of binder (bentonite, or the like), and an adequate amountof CaO.

In FIG. 10, “dry compact” shows that, of 100% sulfur contained in thematerial before reducing-melt, approximately 89% was contained in thecarbonaceous substance (coal) and approximately 11% was contained in theiron ore. When the compacts were subjected to reducing-melt,approximately 85% of sulfur remained in the reduced iron upon completionof the solid reduction explained above with reference to FIG. 4.Approximately 12% of sulfur evaporated and was discharged from thefurnace. When compacts containing no additive CaO source (the calculatedbasicity of the slag component in the composite being 0.165) were used,74.8% of sulfur was trapped in the end-product metallic iron nuggets,and 10.2% of sulfur was trapped in the slag.

When material compacts having their basicity of the slag componentadjusted to 1.15 by adding 3% of a CaO source were used, the amount ofsulfur captured in the metallic iron nuggets decreased to 43.2%, and theamount of sulfur trapped in the slag was increased to 48.8%. The amountof sulfur evaporated and discharged outside the furnace during themanufacturing process reduced to approximately 8%. When materialcompacts having their basicity of the slag component adjusted to 1.35were used by adding 5% of a CaO source, the amount of sulfur captured inthe metallic iron nuggets decreased to 18.7%, and the amount of sulfurtrapped in the slag was increased to 78.8%. The amount of sulfurevaporated and discharged outside the furnace during the manufacturingprocess was reduced to 1.5%.

The above basic experiments using a box furnace demonstrated that thebasicity adjustment by adding a CaO source was particularly effective inreducing the amount of sulfur contained in the metallic iron. The sameexperiment was conducted using a demonstration reactor. In theexperiment, the effect of the basicity on the sulfur reduction of themetallic iron nuggets was quantitatively examined by varying the amountof the CaO source to yield different slag basicities. The results areshown in FIG. 11.

This graph illustrates the relationship between the final basicity ofthe slag and the sulfur content in the metallic iron nuggets. In theexperiment, the slag was produced while varying the amount of the CaOsource, and each of the points in the graph shows an actual result. Theshaded region in the graph shows the results of the above-describedbasic experiments using a box furnace. Since the basic experimentsemployed an electrical heating method and used an inert gas as anatmosphere gas, the oxidation potential of the atmosphere was low, whichadvantageously affects the apparent desulfurization ratio. In contrast,the demonstration furnace employed burner combustion, and thus thereducing degree of the atmosphere gas was low due to the generation ofcombustion gas compared to that of the basic experiments. The sulfurcontent in the metallic iron nuggets was higher than the results of thebasic experiments. However, the basic tendency was substantially thesame as that shown by the results of the basic experiments. It could beconfirmed that when no CaO source was added, the sulfur content in themetallic iron nuggets in the region A was approximately 0.12%. When thebasicity was adjusted to approximately 1.0, the S content was reduced to0.05 to 0.08%, as shown in region B, and the apparent desulfurizationratio was approximately 33 to 58%. When the basicity was increased to1.5, the sulfur content in the metallic iron was reduced toapproximately 0.05%, as shown in region C.

When a CaO source is added to increase the basicity of the slag to 1.8or more, the melting point of the produced slag increases, and theoperating temperature must thus be increased to an excessively highlevel. As a result, the damage on the furnace is accelerated, and theheat economy is degraded. Moreover, the cohesion property of the reducediron is degraded, and the resulting metallic iron is obtained as minuteparticles smaller than 1 mm having a low product value.

As is apparent from these experiments, when an adequate amount of a CaOsource is intentionally added to the material compacts to increase thebasicity of the slag component to approximately 0.6 or more, theproduced slag captures a significantly larger amount of sulfur, and theamount of the sulfur captured in the metallic iron nuggets can thus besignificantly reduced. As a result, metallic iron nuggets that satisfythe level of the sulfur content required in the present invention, i.e.,metallic iron nuggets having a sulfur content of 0.08% or less, can beeasily manufactured. Furthermore, as described above with reference toFIG. 10, the amount of sulfur discharged outside the furnace as SO_(X)or the like during a series of metallic iron nuggets manufacturing stepscan be drastically reduced. Thus, air pollution due to effluent gas canbe minimized. Moreover, load of desulfurizing the effluent gas can besignificantly reduced if desulfurization treatment of the effluent gasis performed.

When the CaO source is added to reduce the S content, as describedabove, bleeding of low-melting point slag which leads to dissolution ofthe hearth refractories may occur during the reducing-melt period due toa decrease in the melting point of the by-product slag depending on theamount of the CaO source added. In implementing the above-describedprocess, a two-stage heating method including a solid reduction periodand a carburization, melting, and cohesion period is preferablyperformed. During the solid-reduction period, the temperature ispreferably adjusted to 1,200 to 1,400° C., and during the carburization,melting, and cohesion period, the temperature is preferably adjusted to1,350 to 1,500° C. In this manner, the solid reduction can besufficiently performed below the melting point of the by-product slag,and, subsequently, the reduction of the remaining FeO, andcarburization, melting, and cohesion of the reduced iron can beperformed to minimize undesirable bleeding of the by-product slag.

In making metallic iron by first solid-reducing material compactscontaining iron ore and a carbonaceous substance and then carburizing,melting, and cohering the resultant material, the amount of thecarbonaceous reductant in the material compacts, the temperatureconditions during solid reduction, and the composition of the atmospheregas and the temperature conditions during carburization and melting, andthe like should be suitably adjusted. In this manner, reduction,carburization, melting, cohesion, and incorporation can be efficientlyperformed, and metallic iron nuggets having a high Fe purity, a suitablecarbon content, and a suitable diameter can be obtained. Under theseconditions, the resulting metallic iron nuggets have a Si content of0.02 to 0.5%, and a Mn content of less than 0.3%. The sulfur content ofthe metallic iron nuggets can be reduced by intentionally adding CaO inthe material compacts so as to adjust the basicity of the slagcomponent.

The resulting metallic iron nuggets of the present invention have a highFe purity, a suitable carbon content, a uniform shape, and a size of 1to 30 mm. Thus the metallic iron nuggets of the present inventionexhibit high handling quality and can thus effectively used as an ironsource for making iron, steel, or various alloy steels.

EXAMPLES

The present invention will now be described in detail using examples.These examples do not limit the scope of the present invention. Variousmodifications are possible without departing from the scope of theinvention described herein. These modifications are included in thetechnical scope of the present invention.

Example 1

Material compacts having a diameter of approximately 19 mm were made byuniformly mixing hematite ore, i.e., an iron source, coal, and a smallamount of a binder (bentonite). Metallic iron was made using thesematerial compacts. The material compacts were fed inside a reducing-meltfurnace of a rotary hearth type shown in FIGS. 1 to 3, and solidreduction was performed at an atmosphere temperature of approximately1,350° C. until a metallization ratio of approximately 90% was reached.Subsequently, the resulting material compacts were transferred to acarburization, melting, and cohesion zone at an atmosphere temperatureof 1,440° C. so as to perform carburization, melting, and cohesion, andto separate by-product slag to make slag-free metallic iron nuggets.

In this process, coal powder, i.e., an atmosphere adjustor, having adiameter of 2 mm or less was bedded on a hearth to a thickness ofapproximately 5 mm before the material compacts were fed to the furnaceso as to control the reducing degree of the atmosphere gas during thecarburization, melting, and cohesion period in the range of 0.60 to0.75. The material composition, the composition of the reduced ironafter completion of solid reduction, the composition of the end-productmetallic iron, the composition of the produced slag, etc., are shown inFIG. 12.

The metallic iron that had been melted, cohered, and substantiallycompletely separated from the slag was then transferred to a coolingzone to be cooled to a temperature of 1,000° C. and solidified, and wasdischarged outside the furnace with a discharger. The production ratiosand the compositions of the recovered metallic iron nuggets, theby-product slag, and the excess carbonaceous substance were analyzed.The reduced iron immediately before the carburization and melting wassampled from the reducing-melt furnace to analyze the composition of thereduced iron immediately before the carburization and melting. Theresults demonstrated that the metallization ratio was approximately 90%,and the residual carbon content was 4.58%. The time taken from feedingof the material compacts to discharging of the metallic iron wasremarkably short, i.e., approximately 9 minutes. The resulting metalliciron had a carbon content of 2.88%, a Si content of 0.25%, and a Scontent of 0.165%. The resulting metallic iron could be easily separatedfrom the by-product slag. A photograph of the produced metallic ironnuggets is shown in FIG. 13. The metallic iron nuggets had a diameter ofabout 10 mm and a substantially uniform size.

Example 2

Material compacts having a diameter of approximately 19 mm were made byuniformly mixing magnetite ore, i.e., an iron source, coal, a smallamount of a binder (bentonite), and 5% of CaCO₃ as a slag basicityadjustor and forming the resulting mixture into compacts.

The material compacts were fed on a bed of coal powder (averagediameter: approximately 3 mm) having a thickness of approximately 3 mm,the bed of coal powder being formed on a hearth. The coal powder wasused as an atmosphere adjustor. The solid reduction was performed as inExample 1 while maintaining the atmosphere temperature at approximately1,350° C. until the metallization ratio reached nearly 100%.Subsequently, the resulting material compacts were transferred to amelting zone maintained at 1,425° C. so as to perform carburization,melting, cohesion, and separation of by-product slag so as to makeslag-free metallic iron. The material composition, the composition ofthe reduced iron after completion of solid reduction, the composition ofthe end-product metallic iron, the composition of the produced slag,etc., are shown in FIG. 14.

The metallic iron that had been melted, cohered, and substantiallycompletely separated from the slag was then transferred to a coolingzone to be cooled to a temperature of 1,000° C. and solidified, and wasdischarged outside the furnace with a discharger. The production ratiosand the compositions of the recovered metallic iron nuggets, theby-product slag, and the excess carbonaceous substance were analyzed.The reduced iron immediately before the carburization and melting wassampled from the reducing-melt furnace to analyze the composition of thereduced iron immediate before the carburization and melting. The resultsdemonstrated that the metallization ratio was approximately 92.3%, andthe residual carbon content was 3.97%.

The time taken from feeding of the material compacts to discharging ofthe metallic iron was remarkably short, i.e., approximately 8 minutes.The resulting metallic iron had a carbon content of 2.10%, a Si contentof 0.09%, and a S content of 0.07%. Since a CaO source was added todecrease the S content in this example, the S content was lower thanthat in Example 1. A photograph of the produced metallic iron nuggets isshown in FIG. 15, and 98% or more of the iron nuggets had a diameter inthe range of 5 to 30 mm.

In this example, because the melting point of the by-product slag wasdecreased due to the addition of the CaO source, bleeding of the moltenslag was feared during the latter period of the solid reduction.However, the example employed a two-stage heating process in which thetemperature during the solid-reduction period was adjusted to 1,200 to1,400° C. to produce reduced iron having a high metallization ratio bysolid reduction, and then the resulting reduced iron was heated at 1,350to 1,500° C. Moreover, because the coal power, i.e., the atmosphereadjustor, was bedded on a hearth, a problem of dissolution of hearthrefractories due to bleeding of molten slag never occurred.

The microscopic structure of the reduced iron at the end stage of thesolid reduction was examined in detail. In Example 1 not using a CaOsource, Fe—(Mn)—S was present on the surface of the reduced iron at ahigh concentration. It was confirmed that during the carburization andmelting, Fe—(Mn)—S was captured inside the molten iron. In contrast, inExample 2 using a CaO source, most sulfur was allowed to react with theCaO source and was fixed during the end stage of the solid reduction. Itwas confirmed that sulfur was prevented from entering the molten ironduring the step of carburization and melting.

Another experiment was conducted as in the above-described experimentbut by replacing the coal powder used as the atmosphere adjustor tofine-particle coal powder, having a particle size of 2.0 mm or less. Itwas confirmed that the S content in the resulting metallic iron wasdecreased to 0.032%.

Example 3

An experiment was conducted under the same conditions as those inExample 1 and an actual furnace. In this experiment, the diameter of thematerial compacts (pellets) was varied within the range of 3 to 35 mm toexamine the effect of the size of the material compacts on the averagediameter and the average mass of the resulting metallic iron nuggets.The results are shown in FIG. 16.

As is apparent from this graph, metallic iron nuggets having a diameterin the range of 5 to 20 mm, i.e., the type of metallic iron nuggetsexhibiting superior handling quality as the end-product metallic iron,could be effectively manufactured from material compacts (dry pellets)having a diameter of approximately 10 to 35 mm.

INDUSTRIAL APPLICABILITY

The present invention having the above-described configuration providesmetallic iron nuggets having a high Fe purity, an adequate C content,and a suitable size for handling ease. The metallic iron nuggets furtherhas low S, Si, and Mn contents, are easy to handle as an iron source,and has a reliable quality. As described above, these metallic ironnuggets can be efficiently and reliably manufactured with a highreproducibility by suitably controlling the manufacturing conditions.

1-7. (canceled)
 8. A method of making metallic iron nuggets, the methodcomprising mixing together a carbonaceous reductant and a metal oxide toform a mixture; and heating the mixture to produce the metallic ironnuggets, wherein almost all of the metallic iron nuggets produced by theheating of the mixture have a diameter in a range of from 1 to 30 mm andcomprise at least 94 mass % of metallic Fe and 1.0 to 4.5 mass % of C.9. The method according to claim 8, wherein 98% or more of the metalliciron nuggets produced by the heating of the mixture have a diameter in arange of from 1 to 30 mm and comprise at least 94 mass % of metallic Feand 1.0 to 4.5 mass % of C.
 10. The method according to claim 8, whereinall of the metallic iron nuggets produced by the heating of the mixturehave a diameter in a range of from 1 to 30 mm and comprise at least 94mass % of metallic Fe and 1.0 to 4.5 mass % of C.
 11. The methodaccording to claim 8, wherein the metallic iron nuggets further comprise0.20 mass % or less of sulfur.
 12. The method according to claim 8,wherein the metallic iron nuggets further comprise 0.02 to 0.5 mass % Siand less than 0.3 mass % Mn.
 13. The method according to claim 8,wherein the heating reduces the metal oxide in a solid state to form areduced iron; and the method further comprises heating the reduced ironin a reducing atmosphere to carburize and melt the reduced iron.
 14. Themethod according to claim 8, further comprising adding a CaO source tothe mixture to adjust the basicity (CaO/SiO₂) of a slag component in themixture to 0.6 to 1.8.
 15. The method according to claim 8, wherein themetallic iron nuggets further comprise less than 0.08 mass % of sulfur.16. The method according to claim 8, further comprising adjusting anamount of the carbonaceous reductant in the mixture so that the residualcarbon content at a metallization ratio of 100% after reduction byheating is 1.5 to 5.0%.
 17. The method according to claim 10, whereinthe metallic iron nuggets have a uniform diameter.
 18. The methodaccording to claim 8, wherein 98% or more of the metallic iron nuggetsproduced by the heating of the mixture have a diameter in a range offrom 5 to 30 mm and comprise at least 94 mass % of metallic Fe and 1.0to 4.5 mass % of C.